3. The database is not the center.  It is hexternal.  

 Externalizing the database can be quite a change for some people used to thinking about applications as “database applications”.  With Onion Architecture, there are no database applications.  There are applications that might use a database as a storage service but only though some external infrastructure code that implements an interface which makes sense to the application core.  Decoupling the application from the database, file system, etc, lowers the cost of maintenance for the life of the application.